The camber angle of a vehicle's wheel is the inclination of the wheel's plane of rotation from a normal to the running surface of the vehicle: for instance the inclination of that plane from vertical when the vehicle is on a horizontal surface. Positive camber is present when the upper part of that plane is outboard of the lower part of that plane: that is when the upper part of the wheel is inclined outwardly from the body of the vehicle. Negative camber is present when the upper part of that plane is inboard of the lower part of that plane: that is when the upper part of the wheel is inclined inwardly towards the body of the vehicle.
In most vehicles the camber angle of a wheel might vary slightly with suspension travel in bump or rebound and—in the case of a steerable wheel—with steering angle, but is otherwise essentially fixed. This effect can result in a reduction in grip when the vehicle corners. FIG. 1 illustrates a vehicle with essentially fixed camber cornering towards the right of the figure. The body 1 is rolled towards the outside of the corner, compressing the suspension on the outer side 2 of the vehicle and extending the suspension on the inner side 3 of the vehicle. This results in the load of the vehicle being concentrated on the edges 4 of the wheels that are closer to the outer side of the corner. As a result of this effect, and other effects arising from the absence of camber freedom, the vehicle may experience a reduction in grip in the corner.
One way to address this problem is for wheels to be mounted on wishbones of unequal lengths so that as a wheel undergoes bump travel its camber changes significantly. However, because the suspension can travel in bump for reasons other than cornering: for example due to unevenness in the road or due to dive under braking, this linkage between suspension travel and camber can adversely affect other aspects of the vehicle's handling.
It has been suggested that it would be desirable for the wheels of a vehicle to be movable in camber as the vehicle corners, independently of bump travel, with a view to maintaining a more even load across the wheel's periphery and thereby improving grip. This would require the wheels on the outer side of the corner to reduce in camber (i.e. towards increasing negative camber) and the wheels on the inner side of the corner to increase in camber (i.e. towards increasing positive camber) as the vehicle enters the corner. Several ways of achieving this have been proposed. In U.S. Pat. No. 3,497,233, lower suspension control arms are mounted on pivoted drop links, and roll of the vehicle body is transmitted to those drop links to cause the camber of the wheels to alter. In the “Load-Sensitive Camber Control Mechanism for a Vehicle Suspension 1992” described at <http://www.malicky.com/davidm/projects/projects.html>, a wheel is arranged on a linkage such that the wheel has freedom to move in camber about a virtual pivot point. Because that point is below the road surface the lateral cornering force on the contact patch of the wheel causes the wheel to move in negative camber when on the outer side of a corner and vice versa. In U.S. Pat. No. 6,776,426, a wheel carrier is mounted on a trapezoidal element that can rotate relative to the body of the vehicle about a substantially vertical axis. The configuration of the trapezoid provides the wheel with a virtual pivot point for camber that can be below the ground.
For a suspension system to be usable in a real-world vehicle it should preferably meet a host of practical requirements. For example, it should not occupy too much space either in the wheelarch or in the remainder of the vehicle; it should allow the desired range of motion of the wheel in bump/rebound, steer and, if desired, camber without parts of the suspension clashing with each other; and it should leave room for the other components that are to be associated with the wheel such as a brake rotor, a brake caliper, control lines and sensors. In practice it has proved difficult to satisfy some or all of these requirements with some prior designs of suspension that offer freedom of movement in camber.